In spring of 2007 a fleet of satellites and ground-based stations (including northern Canadian stations located in Prince George and Whitehorse) caught a powerful space substorm over the northern hemisphere, which revealed something neat about our northern lights. A space substorm is literally a stream of energy from the sun that interacts with the Earth’s magnetic field. University of California researcher, Vassilis Angelopoulos commented that “the auroras surged westward…crossing 15 degrees of longitude in less than one minute. The storm traversed an entire polar time zone, or 400 miles, in 60 seconds flat.” According to CanWest reporter, Randy Boswell (Vancouver Sun, Dec. 12, 2007), researchers had theorized that colossal magnetic columns ran between Earth and the sun, offering a “conduit for energy” that manifests as bursts of northern lights.
NASA scientists call these energy conduits “magnetic ropes”. Solar winds surf them, providing energy for geomagnetic storms and auroras. The researchers described the “ropes” as “a twisted bundle of magnetic fields organized much like the twisted hemp of a mariner’s rope.”
The NASA project in charge of this research is called THEMIS for Time History of Events and Macroscale Interactions during Substorms.
Auroras happen when charged particlesfrom the magnetosphere—mostly electrons but also protons and heavier particles—collide with atoms and molecules of the Earth's upper atmosphere (at altitudes above 80 km). Most of these particles originate from the sun and arrive in a relatively low-energy solar wind. When the trapped magnetic field of the solar wind is favourably oriented (mostly southwards) it reconnects with that of the earth and solar particles then enter the magnetosphere and are swept to the magnetotail. Further magnetic reconnection accelerates the particles towards earth.
These atmospheric collisions electronically excite atoms and molecules in the upper atmosphere. The excitation energy can be lost by light emission or collisions. Most auroras are green and red emissions from atomic oxygen. Molecular nitrogen and nitrogen ions produce some low level red and very high blue/violet auroras (Wikipedia). Typically, an aurora appears either as a diffuse glow or as "curtains" that extend more or less in an east-west direction. At times, they form "quiet arcs"; at others ("active aurora"), they evolve and change constantly. Each curtain is made of many parallel rays, each lined up with the local direction of the magnetic field lines, suggesting that aurora are shaped by the earth's magnetic field. Satellites show that electrons are guided by magnetic field lines, spiraling around them while moving earthwards. The curtains often show folds called "striations", which are curtain-like. When the field line guiding a bright auroral patch leads to a point directly above the observer, the aurora may appear as a "corona" of diverging rays, an effect of perspective.
Here’s how it works: The earth is constantly immersed in the solar wind, a rarefied flow of hot plasma (gas of free electrons and positive ions) emitted by the sun in all directions, a result of the million-degree heat of the sun's outermost layer, the solar corona. The solar wind usually reaches Earth at a velocity of 400 km/s, density around 5 ions/cc and magnetic field intensity around 2–5 nT (nanoteslas; the earth's surface field is typically 30,000–50,000 nT). During magnetic storms, flows can be several times faster; and the interplanetary magnetic field (IMF) may also be much stronger.
The IMF originates on the sun, related to sunspots, and their field lines (lines of force) are dragged out by the solar wind. That alone would tend to line them up in the sun-earth direction, but the rotation of the sun skews them (at Earth) by about 45 degrees, so that field lines passing Earth may actually start near the western edge ("limb") of the visible sun.
The earth's magnetosphere is the space region dominated by its magnetic field. It forms an obstacle in the path of the solar wind, causing it to be diverted around. At a distance of about 70,000 km (before it reaches that boundary, typically 12,000–15,000 km upstream) a bow shock forms. The width of the magnetospheric obstacle is typically 190,000 km, and on the night side a long "magnetotail" of stretched field lines extends to great distances.
When the solar wind is “perturbed”, it transfers energy and material into the magnetosphere. The electrons and ions in the magnetosphere that become energized move along the magnetic field lines to the polar regions of the atmosphere.
The aurora borealis is named after the Roman goddess of the dawn, Aurora, and the Greek name for north wind, Boreas.
Both Jupiter and Saturn have magnetic fields much stronger than Earth's and both have large radiation belts. Auroras have been observed on both planets, most clearly with the Hubble Space Telescope. Venus, Mars, Uranus and Neptune also have auroras.
The auroras on the gas giants appear to be powered by the solar wind. In addition, Jupiter's moons, especially Io, are powerful sources of auroras. These come from electric currents along field lines ("field aligned currents"), generated by a dynamo mechanism due to the relative motion between the rotating planet and the moving moon. Io, which experiences active volcanism and has an ionosphere, is a particularly strong source, and its currents also generate radio emissions.
Ilker Yoldas, over at The Thinking Blog, posted some incredible pictures of auroras (both borealis and australis) here. If you want to be further amazed check these images out. Ilker also posted some video so you can catch the dance of Nature's light. Worth it...
There are more things in heaven and earth, Horatio, Than are dreamt of in your philosophy.-- William Shakespeare, "Hamlet", Act 1 scene 5
Nina Munteanu is an ecologist and internationally published author of novels, short stories and essays. She coaches writers and teaches writing at George Brown College and the University of Toronto. For more about Nina’s coaching & workshops visit www.ninamunteanu.me. Visit www.ninamunteanu.ca for more about her writing.